mt76: mt7915: fix ht mcs in mt7915_mcu_get_rx_rate()

The mcs value of HT mode reported by mt7915_mcu_get_rx_rate()
has already been converted to the expected format.

Tested-by: Yiwei Chung <yiwei.chung@mediatek.com>
Signed-off-by: Shayne Chen <shayne.chen@mediatek.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>